Input validation is a fundamental security practice in application and business security that ensures only properly formatted, expected, and authorized data is accepted by an application. Since user input is one of the most common sources of security vulnerabilities, proper validation helps prevent attacks such as SQL injection, cross-site scripting (XSS), command injection, buffer overflows, and data corruption. Input validation should be applied to all external data sources, including web forms, APIs, cookies, files, databases, and network requests. Effective input validation improves application reliability, protects sensitive business information, and enhances overall system security. One of the basic input validation approaches is whitelist validation, also known as positive validation. This approach specifies exactly what type of input is considered acceptable and rejects everything else. Validation rules may define permitted characters, maximum and minimum lengths, numerical ranges, file types, formats, and patterns. For example, a customer identification number may be allowed to contain only digits and have exactly ten characters. Since only expected values are accepted, whitelist validation is considered the most secure and recommended approach for most business applications. Another common approach is blacklist validation, also called negative validation. Instead of specifying acceptable input, it blocks known malicious characters, keywords, or patterns. For example, it may reject special SQL keywords or scripting tags such as `